System for measuring and eliminating impedance variations



G.' H. BAKER ET AL SYSTEM FOR MEASURING AND ELlfMINATING IMPEDANCE VARIATIONS Filed June 14, 1946 March 14, 1950 ATTORNEY Patented Mar. 14, 1950 UNITED STATES PATENT OFFICE SYSTEM FOR MEASURING AND ELIMINAT- ING IMPEDANCE VARIATIONS Application June 14, 1946, Serial No. 676,616

4 Claims. 1

This invention relates to transmission systems and more particularly to testing apparatus and method for use therewith in the measurement of and reduction of Variations in the matching of impedances.

When a wave generator is connected to a load circuit by a transmission line, it is generally desirable that the impedance relation throughout the system be carefully matched. A mismatch of the impedances occasions reflection of the transmitted energy from the point of mismatch back to the wave generator source and if the mismatch be of sufficient magnitude it may seriously influence the operating characteristic of the wave generator. In some particular types of systems, frequency or power variations in the wave source may give rise to serious errors in the resulting process.

In its general embodiment the invention proposes a method and system for observing and minimizing the effects of Variations in the impedance match between a generator and its connected load.

In direction indicating systems employing amplitude modulations of a reflected signal for the determination of the location of an object in space, a shifting power or frequency output of the wave source may introduce a serious error in the indicated direction. It is an object if this invention to effect a reduction in these errors byy making possible more accurate measurements of the variations in the impedance relations and by making possible more accurate adjustments at the boundaries of dissimilar impedance elements.

The invention has a further obiect to make possible quick and accurate determination of the error introduced into a direction indicating system because of variations in the impedance relation between a wave source and its connected load.

The manner in which these and other objects are accomplished may be more clearly understood from the following description of one preferred embodiment with reference to the accompanying drawing in which:

Fig. 1 shows in block diagrammatic form the general arrangement of a radio energy reflection type object locating system which incorporates the principles of this invention;

Fig. 2 shows in longitudinal cross-section the attachment of the directive pick-up element to the coaxial transmission line; and

Fig. 3 shows a cross-sectional end View of the eccentric mounting device used in the reduction of the impedance mismatch.

Apparatus and methods for measuring and matching steady-state impedances have long been known in the art. However, in systems in which one of the impedance elements varies in a cyclic manner it may or may not be possible to secure an exact matching of the impedances and therefore it becomes highly desirable to reduce the variations to a minimum and to minimiz the effect of these variations.

The copending application of Philip H. Smith, Serial No. 498,622, filed August 14, 1943, discloses a directive antenna system utilizing a rotating semi-dipole antenna element. Our eX- perience with this type of antenna system has disclosed a difficulty that may occasionally arise when it is incorporated into a radio energy reflection type object locating device such, for eX- ample, as might be used for anti-aircraft yfire control. As will be explained in further detail, this difficulty arises through a variation in the impedance matching of the component elements.

In such a system the horizontal and vertical location of the target object in space is determined through the variations in amplitude of the reflected energy pulses. The object location is portrayed on the screen of a cathode ray tube on which the position of the paraboloidal reflector axis is indicated by the intersection of cross-hairs. The position of the reflected energy object dot with respect to the intersection of the cross-hair denotes the displacement of the target object from a point in space corresponding to a point on the principal axis of the reflector unit. For a more complete explanation of the operation of such a system, reference should be made to the above-mentioned copending application of P. I-I. Smith and Patent 2,426,182 grantedv on August 26,y 1944 to O. E. DeLange. For the purposes here intended, it is sufcient to point out that if no object is encompassed within the area covered by the transmitted energy beam so as to reflect a portion of that energy, the object dot should coincide with the cross-hair intersection and any factor that under this condition causes a displacement of the dot from this point of coincidence thereby introduces an error that leads to an erroneous direction indication when energy is being returned from a reflecting object.

In using a system such as disclosed in the above applications, we have occasionally experienced this erroneous direction indication and believe it may be caused by cyclic variations in the mismatch of impedances between the wave source and the antenna load device. This mismatch may originate in changes in the antenna element impedance as seen from the wave source during the rotation of the antenna element. Viewed from the wave source end, this changing impedance represents a change in its connected load and tends to pulL or cause the magnetron oscillator to change, its operating irequency, its power output, or both. As the antennakelement is rotating in a cyclic manner, and the impedance mismatch varies in a cyclic manner, the change in the wave source shifts in a like mode and a modulation component, .frequency or amplitude, is imparted to the outgoing transmission energy pulse. The reflected portion oi this modulated energy 'pulse appears as an amplitude or a frequency kmodulated signal to the radar receiving equipment. Because of the characteristic of the bandfpass iilter organization used in the receiving equipment, to secure suitable discrimination Vagainst noise and iinwanted signals, the frequency modulated signal may be converted'into .an amplitude modulated signal. If .an amplitude modulation component -was imparted to the transmitted energy pulse `the reflected pulse will include =a like component. Nor-mal 7operation of the Ireceiving radar equipment is predicated upon the `recei'ptoi` an amplitude modul-ated signal, caused bythe target object Vnot coinciding with Ithe principal 4axis of the reflector unit, and if an extraneous amplitude modulation component is .introduced into this signal, ithe location of the target object will be in error by the displacement contributed by this component. it has :been our experience that this displacement does occur and that it occurs because fof the frequency -or power pull cr modw lation of the magnetron oscillator caused hy variations in the impedance matching within the system.

The basic cause of variations inthe antenna load impedance during the rotational cycle of the antenna element, appears 'to .be the lack of symmetry among the portions of the antenna rsystem. 'This lack of symmetry arises from irregularities produced during the manufacturing and assembling processes which, though harmful, are ywitliin the normal Jtolerances described for this type of equipment. To attempt -to reduce these variations through refined manufacturing procedures appears to involve uneconomical met-hods. Variations ralso arise from 4the necessarily unsymmetrical arrangement of surfaces in effec- Ytive close vproximity to the apparatus when it is used `under ordinary operating conditions.

Variations of the form-er type which are fixed `in relation to the antenna system, that'is, are in a rsense vinherent in the antenna system, may be compensated. Variations of the latter type which are caused by adjacent surfaces not 'in xed relation to the antenna system, such, for example, yas parts of a ship structure, though not compensated bythe invention, form a very minor variation and do not'materially contribute to the displacement error. This latter type of variationv is changed with movement of the antenna system.

To correct these causes of impedance variation without resorting-to uneconomical refinements in the mechanical design, our copending applica tion Serial No. 676,393, illed June '13, 1946, now Patent No. 2,492,951, dated January 3, 1950, discloses means for applying a dynamic impedance adjustment as the antenna element is rotated.

In a direction indicating system such as referred to above in which a paraboloidal reflector Aunit and lan antenna element are employed in fa system. This generator may be of any of the types well known in the art, for example, any of erating circuits. The generated pulses are approximately one-half micro-second in duration and are square-topped to secure the greatest energy content. Energy pulses produced by the transmitter I3 are supplied to the transmission 'line I4 for transmission tothe quarter-wave coupling transformer 15 which couples vthe 'trans- Amitti'ng and receiving equipment to the coaxial line I6, I1 of the antenna system. This coupling and the antenna system generally are explained in greater detail in 'our copending application referred to. In this antenna system a semi-dipole antenna element 2li is mounted on and connected 4to the inner conductor I l of `the coaxial line which comprises the outer conductor lIV'I and passes through the vertex :of the paraboloidal 'reflector I3. The inner yconductor i6 is connected to the motor 24 through an .insulating coupling 23 and is rotated by the motor 24 at ya rate of about 29 revolutions per second. A twophase generator 25 is mounted on `the `motor shaft and is cdriven by the motor Z6 to `.produce a quadrature spaced two-phase output which is supplied to connecting .lines 132 and 43 'for use in resolving the antenna position at any given instant in its cycle. In order to match 'the impedance of 4the senor-dipole antenna element 2) to the comparatively low impedance of the com axial line ycomprising inner conductor It and louter conductor l l, a series of impedance matching transformer elements (not shown on the drawing) are provided within the coaxial line structure. It will be noted that the antenna element '2&3 is mounted on the rotating inner conductor I6 through the use of a solid conically shaped mounting unit i9. A rotating reflector unit 2| is provided between the antenna element 20 and the outer end of the rotating conductor I6 where it is supported in its eccentric mounting 22. The eccentric mounting Z2, as disclosed and claimed in our copending application rreferred to, is adjustable and the entire array comprising the coaxial line, after it passes through thevertex of the paraboloidal reilector It, the conical structure IE, antenna element 2li, se"- ondary reflector vantenna 2i and the Veccentric mounting `22 are housed in a Plexiglas housing vv(not shown herein). The end of the lrotating inner conductor I6 at the right is supported in a bearing which is in turn supported in the eccentric 'mounting 22. As shown inFig. '3, which presents an end cross-sectional view of the mounting structure 22, the end of the rotating `inner conductor I6 is shown in a dotted circle. Numeral 5| indicates an end cap or plug to cover the end of the rotating shaft. Numeral 52 indicates an eccentric mounting mounted within and enclosed by an outer eccentric mounting 53. Locking set screws 54 and 55 are provided for vholding 'the inner eccentric mounting 1.52 in xed relation `with the outer eccentric mounting 53. Numeral 5B indicates the eccentric mounting locking nut which is a circular ring threaded into the cylindrical chamber 51, contained in the antenna end casting 58, and holds the outer eccentric mounting 53 in position. By moving the relative positions of the inner and outer eccentric mountings 52 and 53, respectively, it is apparent that the end of the rotating inner conductor I6 may be variously positioned with respect to the mid-point of the cylindrical chamber 51. This operation effectively changes the longitudinal axis of the rotating inner conductor I6 with respect to the inside of the outer coaxial conductor Il' as well azhangestbc plane of rotation of the antenna element 20 with respect to its adjacent structures; Antenna impedance variations arise from changes in the proximity of the outer surface of the inner rotating conductor I6 and the inner surface of the outer stationary conductor I1 of the Acoaxial line and in particular from changes inthe separation between the outer surface of the conical structure I9 and the inner surface of the outer coaxial conductor I'I which is flared at this point. The flared portion of the outer conductor I1 is one of the factors in the serially connected impedance matching transformer array for conditioning the impedance of the antenna element 2B for matching the impedance ofthe coaxial line. Variations between these surfaces change the impedance matching relation of the impedance elements and when aided by variations in the mechanical separation between the outer surface of the aforesaid flared conductor portion and the antenna element 2t as Well as between the antenna element Z and the secondary reiiector antenna 2 I, give rise to cyclic Avariations in the impedance of the antenna. system which variations cause modulation of the transmitted energy pulse because of fluctuations in the oscillating frequency or power output of the magnetron oscillator included in the transmitting equipment I3.

Reflected energy, whether derived from an impedance mismatch or from a reflecting object is returned to the T--R box 2G by way of the quarter-wave coupling transformer i5 and the coaxial feed line M. This T-R unit 26 is a voltage operated device to act as a relay for connecting the receiving equipment during a receiving period and For instantly disconnecting the receiving equipment from the transmitting vequipmentduring a transmitting period to prevent burn-out or damage to the receiving equipment during the high energy transmission period. One of several well-known types of this equipment comprises a resonant cavity tuned to the frequency of the transmitted pulses and having a gas-filled two-element vacuum. tube connected across points within the cavity of substantially different potentials. When the transymitter is keyed this vacuum tube breaks down kvradar unit and are old elements, the structure 6 and operation of lwhich are well understood in the art.

Pulses from the pulse generator H are rsupplied by way of circuit 33 to the range unit 3l, which unit is well known in the art and provides a continuously adjustable phase shift and hence a continuously adjustable time delay factor. In accordance with the invention, the delayed pulses from the range unit 3! actuate the range step generator 32 which supplies by Way of circuit 34 a synchronizing pulse to the gate pedestal generator 35. The output of the gate pedestal generator 35 serves to unblock or open the gate amplier 38 by virtue of its squaretopped positive voltage wave delivered over circuit 36. The gate amplifier 265i is so biased that it will accept Ionly echo pulses which are coincident in time with the pedestal pulse. Therefore, in normal operation, the desired reflected pulse is derived from the video detector 29 by way oi contact 3l and brush 38 of the switch, as shown, and is superposed on the positive voltage pulse derived from the gate pedestal generator t5 to be passed to the remainder of the circuit. This arrangement provides discrimination for all except the desired pulse. In accordance with the invention, the brush arm 38 is operated to the Contact it, whereby the normal operation of the system is interrupted and the signal input to the gate ampliiier is derived over circuit 'I2 from the silicon crystal detector 1i. Crystal detector il is excited by energy derived over circuit 10 from the directive pick-up 50. The details of this directive pick-up ii and the manner in which it is attached to the coaxial line I4 are shown in greater detail in Fig, 2. This pick-up is substantially the same as that disclosed in the copending application of W. W. Mumford, Serial No. 540,252, led June 14, 1944, and for a detailed description of the operation of this unit reference should be made to that application.

Referring to Fig. 2 it will be noted that two openings S3 and 64, spaced apart by one-quarter wavelength of the operating frequency, are made in the outer conductor 62 oi the coaxial line I4 and the outer conductor i'i of the directive pickup unit 50. When a mismatch of impedances exists standing waves are set up in the transmission line I4 which originate from energy reected at the point of mismatch back toward the generating source. The total energy in the coaxial line Id comprises the transmitted energy from the transmitter i3 and the refiected energy traveling in an opposite direction. Energy from the transmitter I 3 has two points of entry to the directive pick-up unit 6D, one through the opening 53 and another through the opening 64 spaced one-quarter wavelength therefrom. It is apparent that the transmitted energy passing through the opening Gli and received at the silicon crystal detector 'II travels a distance onehalf wavelength farther than the transmitted energy entering through opening 63 and received at detector 1I. The two distinct energies will therefore be in phase opposition and will mutually cancel. Also, energy reiiected from the point of mismatched impedances through the ouarter'- wave coupling transformer I5 reaches the silicon crystal detector 'II through the openings 63 and 6ft. The two reiiected energies passing through the two openings arrive at the detector li in phase agreement since the lengths of the paths traversed by these energies are equal. The amount of the reflected energy received by the Ysilicon crystal detector 'Il will, therefore, vary `as the impedance :mismatch varies during the rotational cycle. The operation of the crystal detector is well known and one of its output components will Abe equivalent to an 1800-cycle pulse `carrier upon which -is superposed a 29-cycle amplitude modulation envelope. This corresponds to an antenna rotational speed of `29 .revolutions per second and a pulse rate of 1800 pulses lper second. This modulated carrier .is supplied to the gate amplifier 39 by way of contact i3 and brush arm 38.

By eliminating the delay in the range unit 3l the gate pedestal generator is operated at a time Icorresponding to the returned reiiected energy received from the crystal and, therefore, the gate amplifier 39 'is conditioned to accept these pulses.l The 1800-cycle pulse carrier with its 29-'cycle Amodulation envelope is passed by the gate ampliiier '39 and is fed to two channels one of which, the automatic gain control channel, does not concern us here. The remaining portion of the amplifier output is fed to the detector vand filter unit 49 where amplitude modulation, if present, is selected by employing ordinary grid 'lea-k detection and ordinary ltering means. This unit passes only frequencies in the immediate vicinity of 29 cycles per second. The output of the detector and filter unit 4Q is limited in amplitude by a simple varistor circuit in the limiter rectifier and phase inverter unit 4i. In this circuit a portion 'of the energy is inverted in phase, that is, changed by 180 degrees, Vby use, for example, of any of the well-known phase inverting vacuum tube circuits whereby from a single voltage input two voltages inverted in phase with respect lto each other are derived. of unit l4l comprising two 180 degrees displaced voltages is applied to the 29-cycle modulator circuit comprising units 44, 45, 46 and 41. These four units comprise a phase and amplitude sensitive device which combines two 25J-cycle voltages, 189 degrees' out of phase, with two other 29-cycle voltages, 90 degrees out of phase. The plate circuits of these units are vequipped with low-pass condenser, resistor type lters so that average, rather than instantaneous, plate potentials are available for use as dellection voltages for the cathode ray tube 48. One portion of The output f 8 t convert this information into voltage component that control the vertical and horizontal deection on the screen of the cathode ray tube 48.

For a condition of no signal, the cathode ray tube is focussed so that its spot coincides with the intersection of the cross-hairs 49.k For a ycondition where no amplitude ,modulationis present on the reiiected energy, there would be no voltage supplied Vby unit 4| and therefore no deiiection of the spot on the cathode ray tube screen. This condition would correspond to one where there was no impedance mismatch between the transmitter i3 andthe antenna system or where if a mismatch did exist there was no variathe output of device 4I is supplied to modulator v units 'rtl4 Iand 4E. The other portion of the output, which is in inverted relation thereto, is supplied to modulator units and 41. The two-phase generator 2e provides an output comprising two 29cycle sine wav-e 90 degrees displaced voltages.

@ne of these voltages is supplied by interconi necting circuit 42 to modulators 44 and 45 while the other voltage is supplied by interconnecting circuit 43 to modulator Iunits 46 and 4?. Modulator units 44 and 45 control the vertical deiiection in the cathode ray tube 48 while modulator units 46 Aand y4'I'control the horizontal deiiection therein. The center of the cathode ray tube screen is indicated by the intersection of 'the cross-hairs 455. The excitation for the phase inverter unit 4i is obtained, as previously discussed, from the reflected energy obtained as the impedance of the antenna element 20 varies `during its rotation. Resolution of the 90 degrees phase displaced output of the two-phase generator 25 with the 180 degrees phase displaced output of the phase inverter unit 4|, indicates the angular position of the antenna element 20 with respect to an arbitrary point of reference at the time of the return of the reflected energy. The 29-cycle modulator units 44 to 4l, inclusive,

tion of this mismatch during the rotationalcycle. N

Therefore, when a signal is supplied which results in shifting of the spot from the center of the cross-hair intersection, it is evident that amplitude `modulation is present in the reected energy.

Since this nuctuating impedance may so affect the magnetron characteristic that amplitude modulation components, or frequency rshifts capable 4of reduction to amplitude components,

-r are `imparted to the energy pulse and hence to the reflected pulse it is clear that a shift of the radar dot on the cathode ray tube screen may be superimposed on the rdeflection normally produced by energy returned from a target object. If this error :is the result of frequency variations, its magnitude will be affected by the tuning of the receiving equipment.

To eliminate this impedance variation, suitable adjustment is made in the axis of rotation of the inner conductor i5 .and the antenna semidipole element 2th The antenna housing (not shown) for the bearing and eccentric mounting assembly is vremoved and the locking ring 56 (Fig. 3) is loosened as are locking set screws 54 and 55, this double Yeccentric bearing being shown more fully in our copending application referred to. The equipment is turned on and the antenna element 2i] is rotated at its normal speed, with switch arm 38 and contact 'i3 engaged. Initially, the output of the gate amplifier 39 is grounded through the switch 5@ so that no reected signal is applied to the detector lter unit 40. Under these conditions the object dot should coincide with the intersection of the cross-hairs 49. The reflected signal is then passed to the detector iilter unit by restoring switch 50 to the unoperated position and any shifting of the object dot may be observed on the cathode ray tube screen. The eccentric units '52 and 53 are then adiu'sted as follows. Using small rods which t holes in the eccentric units, hold one in its existing position and rotate the other to a directly opposite position. Visualize a straight line passing through the off-center position of the object dot and the center of the rcross-hair intersection as observed on the screen of the cathode ray tube 48. Keeping the direction of this imaginary line in mind rotate both eccentrics as a unit until the two handles make a line which would be approximately perpendicular to this imaginary line. Now, if both handles are rotated through the same angle but in opposite directions the object dot should move along the straight line either toward or away from the center. Proper adjustment of the eccentric unit to center the dot will have to be determined by trial while .observing the shifting of the spot on the cathode ray tube screen. For any stationary position of the antenna system, when no reflected energy isbeing returned from an extraneous object, ,it

' the axis of rotation of the inner rotating conductor I6 and the antenna element 20 that varia-A tions in the separations between the surfaces of these units and adjacent surfaces during the rotational cycle are eiectively counter-balanced and no Lnet impedance variation occurs during the cycle. As previously mentioned, it should be borne in mind that some slight and inconsequential variation may be introduced from surrounding structures when the antenna position is shifted from that which it occupied during the adjustment period. Locking set screws 54 and 55 are "now tightened to position the eccentric units 52 and 53 in their iinal positions and locking ring 56 is tightened to retain the outer eccentric unit 53 in its iinal position. Return of the switch arm 38 to the contact 31 restores the unit for normal operation.

Although the above embodiment shows the invention as incorporated in a radio energy refleeting device used for object location, it should be appreciated that its scope should not be so limited. Other applications within the scope of this invention will doubtlessly occur to those skilled in the art.

What is claimed is:

1. A method of eliminating, in a system comprising an impedance element connected to a source of energy and rotating about a given axis, the impedance variation of said element produced during rotation of the element, which comprises extracting from said system a current having an amplitude and phase representing the cyclic variation in impedance of said element during its rotation, determining the amplitude and phase of said current, and shifting said axis of rotation an amount dependent upon said amplitude and in a direction dependent'I upon said phase.

5. A method, in a system comprising a coaxial line connected to a transmitter and comprising a stationary outer conductor and a rotating inner conductor, a linear antenna element attached to the end of said inner conductor, of eliminating a variation in the impedance of said element produced during its rotation, utilizing a source of reference voltage and means for shifting the axis of rotation of said element and inner conductor, which comprises selecting the energy reected by said element, detecting from said reiiected energy a component having a phase and amplitude representing said impedance variation, comparing said component with said reference voltage to determine the phase and amplitude variation thereof, and shifting the axis of rotation of said element and inner conductor relative to said stationary outer conductor an amount dependent upon the phase and amplitude of said component. l

3. In a system comprising a rotating impedance element, a source of reference voltages, a source of energy, a transmission line connecting said source to said rotating impedance element, said transmission line comprising a coaxial line having a stationary outer conductor and a rotating inner conductor attached to said impedance ele ment, a method of measuring and eliminating the variations in the impedance of said rotating impedance element which comprises selecting the energy reflected by said rotating impedance element, detecting from said selected energy an amplitude modulated energy wave corresponding to said variations, comparing said detected energy with said reference voltages to determine the phase and amplitude variations thereof, and shifting the axis of rotation of said inner conductor relative to said outer conductor an amount and in a direction dependent upon the amplitude and phase of said variations.

4. In a radio direction indicating system, a source of radio energy, a stationary parabolic reflector, a linear antenna element positioned in front of said reflector, a coaxial line extending through the vertex of said reiiector and comprising a rotating inner conductor connected to said element and a stationary outer conductor, means connected to said line and comprising a directive pick-up and an indicator for ascertaining and evaluating any variation in the impedance of said element produced during its rotation, and means for compensating for said variation, said means comprising a double eccentric bearing supporting said inner conductor for adjusting `the position of said rotating inner conductor and associated rotatingr element relative to said stationary outer conductor.

GEORGE H. BAKER.

ELM() E. CRUMP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,853,953 Becker Apr. l2, 1932 2,256,787 Lazar Sept. 23, 1941 2,403,562 Smith July 9, 1946 2,412,612 Godet Dec. 17, 1946 2,417,248 Godet Mar. 11, l1947 2,423,390 Korman July 1, 1947 2,424,193 Rost et al July 15, 1947 2,446,024 f Porter et al. July 27, 1948 

